The Indian River Lagoon system (IRL) is a body of water on the central east coast of Florida. It is approximately 155 miles long, delineated by Ponce de Leon Inlet to the north and Jupiter Inlet to the south (Fig. 1). More importantly, the IRL extends from latitude 29°03’N to 26°57’N. This length of just over 2 degrees latitude straddles the divide between the temperate Carolinean Province north of Cape Canaveral, and the sub-tropical Caribbean Province to the south. This change in latitude has a tremendous effect on species diversity and species composition at either end of the IRL (Woodward and Clyde, 1994).

The IRL is composed of three lagoons: the Indian River Lagoon, Banana River Lagoon, and Mosquito Lagoon. The portions of the Indian River Lagoon south of St. Lucie inlet are commonly referred to as Hobe Sound and Jupiter Sound. Due to its long narrow morphology, more than two thirds of the IRL lies beyond the greatest tidal excursion distance, suggesting limited direct exchange between the IRL and the Atlantic (Woodward and Clyde, 1994).

There is significant exchange of water between the three lagoons. In the Banana River and Mosquito Lagoons, evaporation and seepage through the porous sands of the barrier island into the Atlantic exceed inputs from surface flow and rainfall. This indicates that the Banana River Lagoon acts as a sink for water from the Indian River Lagoon, and Mosquito Lagoon acts as a sink for the Indian River Lagoon through the Banana River Lagoon (Woodward and Clyde, 1994). This large exchange with the Atlantic by seepage is one of several factors that distinguish the lagoon from the classical definition of an estuary.

The IRL cannot be properly thought of as just the water body; it must be considered as the lagoon complex and the watershed that supplies its water. A watershed can be described as all the land that water falls on, flows over, through, and under on its way to some receiving body of water. The IRL watershed has been heavily impacted by human activities.